The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell power plant that utilizes a water immiscible fluid having a low freezing temperature to displace a water coolant within fuel cells and a coolant loop of the plant.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing mechanical damage when the product water and/or any water coolant fluid freezes, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is shut down in sub-freezing conditions.
Accordingly there is a need for a fuel cell that may be shut down in sub-freezing conditions that does not sustain mechanical damage resulting from freezing and that does not require isolating an antifreeze cooling fluid from the cathode and anode electrodes within a sealed coolant system so that fuel cell generated product water may be removed within porous water transport plates.
The invention is a freeze tolerant fuel cell power plant that includes at least one fuel cell for generating electrical current from reducing fluid and process oxidant reactant streams; a coolant loop having a coolant pump that directs a water coolant through a coolant passage, a water transport plate within the fuel cell, a coolant heat exchanger, and back to the fuel cell; and a water displacement system having an open tube accumulator that contains a water immiscible fluid and water coolant, and having a water immiscible fluid pump that pumps the water immiscible fluid from a discharge of the accumulator through a water immiscible feed line to displace the water coolant within the coolant loop. Also included in the water displacement system is a heater secured to the water immiscible feed line to heat the water immiscible fluid passing through the feed line, and a coolant loop drain line secured between the coolant loop and an accumulator inlet for draining the water coolant and/or the water immiscible fluid from the coolant loop into the accumulator. The system also includes displacement valves for selectively directing the water immiscible fluid to flow from the accumulator into the coolant loop, for selectively directing the water coolant to flow into the accumulator, and for selectively directing heated water immiscible fluid from the feed line back into the accumulator to heat water coolant in the accumulator. The water displacement system may also include a water immiscible fluid re-cycle line secured downstream of the heater between the feed line and the inlet of the accumulator to direct heated water immiscible fluid to the inlet of the accumulator.
The open tube accumulator includes a plurality of open plastic tubes that are configured as a heat exchanger. Liquid phase water coolant surrounds exterior surfaces of the open plastic tubes, and upon freezing of the water coolant within the accumulator during a long term power plant shut down, the plastic tubes deform to absorb a volume increase of the freezing water to avoid mechanical damage to the accumulator. Upon start-up after the long term shut down, the water immiscible fluid re-cycle line directs heated water immiscible fluid from the discharge of the accumulator and the heater into the inlet of the accumulator so that the heated water immiscible fluid flows through the open tubes to thaw the water coolant.
In a preferred embodiment, the water immiscible fluid is selected from the group consisting of perfluorocarbons, hydrofluoroethers, alkanes, alkenes and alkynes. Exemplary water immiscible fluids include straight chain alkanes such as octane, nonane and decane and mixtures thereof. The water immiscible fluid may have a density that is greater than or less than the density of water. A preferred density differential of the water immiscible fluid compared to water is plus or minus 0.2 grams per cubic centimeter.
In use of the freeze tolerant fuel cell power plant during normal operation, the water immiscible fluid remains within the open tube accumulator separated from any water coolant within the accumulator, and water coolant cycles through the fuel cell and coolant heat exchanger to maintain the fuel cell within an optimal temperature range. When the fuel cell power plant is shut down for a short term shut down, the displacement valves operate to control flow of the water coolant into the accumulator, and the water immiscible pump directs the water immiscible fluid into the coolant loop to displace the water coolant. The heater may be used in conjunction with the water immiscible pump to provide heated water immiscible fluid through the fuel cell to maintain the fuel cell temperature above a minimum level. When a desired temperature is achieved, the water immiscible fluid is directed back into the accumulator. To return the fuel cell power plant to operation after such a short term shut down, the coolant pump is utilized to direct water coolant from the accumulator back into the coolant loop.
For a long term shut down, the same procedure is undertaken by the displacement valves to direct the water coolant into the accumulator; to direct the water immiscible fluid into the coolant loop to displace the water coolant; and, to then drain the water immiscible fluid back into the accumulator. Periodic heating by the water immiscible fluid is not undertaken, and water coolant in the accumulator and/or within pores of fuel cell components is permitted to freeze. To start up the power plant after a long term shut down, the displacement valves first direct the water immiscible fluid to pass from the accumulator discharge through the heater and the re-cycle line to pass into the accumulator inlet and to flow through the open tubes of the accumulator to melt frozen water coolant. Next, the displacement valves direct the heated water immiscible fluid to pass through the coolant loop to melt any ice within the water transport plate and any other fuel cell components. Then the water immiscible fluid is directed back into the accumulator while the water coolant is directed into the coolant loop so that the fuel cell may commence generating electrical current.
The coolant loop may also include a gas separator to direct any reactant gas out of the coolant loop, and the gas separator may include an overflow line to direct excess product water into the accumulator whenever the power plant is producing more water than it is utilizing, which is characterized as being in positive water balance. In the event the power plant is operating in negative water balance, water may be directed from the accumulator to supplement water coolant in the coolant loop.
Accordingly, it is a general purpose of the present invention to provide a freeze tolerant fuel cell power plant that overcomes deficiencies of the prior art.
It is a more specific object to provide a freeze tolerant fuel cell power plant that permits utilization of a porous water transport plate that facilitates removal of product water from fuel cells of the plant.
It is yet another object to provide a freeze tolerant fuel cell power plant that provides for rapid start up of the power plant after a short term shut down.
It is another object to provide a freeze tolerant power plant that prevents mechanical damage of the plant by freezing of a water coolant during long term shut down of the power plant.
These and other objects and advantages of the present freeze tolerant fuel cell power plant will become more readily apparent when the following description is read in conjunction with the accompanying drawings.